Support JupyterLab 3

[bollwyvl]

As of posting, JupyterLab 3 is still a release candidate, but is a pretty compelling way forward, and already pretty easy to develop against pip or conda. As #10 adds a serverextension to handle the .wasm mime type junk, it would be a relatively small step to move to a pip install-and-there-you-go end user experience.

Some concrete steps on top of #10:

• migrate to a jupyter_server (vs notebook) extension...
  • I haven't done this before, but it can't be that different
• add the labextension build stuff, dumping into src/jupyterlab_markup/static
• redo the build plumbing
  • source the version (and other package information) only from static/package.json
  • I'm a fan of doit for keeping the package in a releasable state from a single package

Because a big part of #10 is to make this extensible, it would still be important to do npm releases, so that others can get at the Token to make extensions with additional markdown-it plugins.

[agoose77]

@bollwyvl could you elaborate more on doit / suggest any resources for learning about it? I've seen it around, but never had hands on experience.

[agoose77]

Closing as most of this work has been done now :)